Dietary fat ratio's influence on adolescent depression: A nonhuman primate model Summary Diets high in saturated fats impact metabolic function in children and may predispose them to the development of mental illnesses, such as depression and other affective (mood) disorders. This is suggested by the increased rate of depressive illness diagnoses in children that has occurred simultaneously with an increased incidence of childhood obesity and metabolic disorders. Several studies have linked consumption of excess saturated fats with alterations in cognitive performance, and decrements in social behavior, activity levels and reward processing, all common features of adolescent depression. Controlled mechanistic studies of the effects of fat intake on affect, however, have been difficult to conduct in human adolescents. Here we propose to use a small nonhuman primate, the common marmoset, Callithrix jacchus, to study the relationship between dietary fatty acid ratios and key components of adolescent depression including problem solving, learning, social behavior, activity levels, and reward processing. We have previously shown that this family-oriented, cooperative breeding species shows improved glucoregulatory function with an increasing dietary ratio of unsaturated to saturated fats. We propose to expand these studies to test the hypothesis that increasing the amount of unsaturated fats (essential fatty acids) consumed in the form of healthy snacks will improve problem solving and learning, activity levels and positive social interactions in adolescent marmosets whereas increasing the amount of saturated fats will produce a negative effect on these measures. We will use 48 marmosets in 3 groups of 16 animals (8 males and 8 females per group) each. Groups 1 and 2 will be fed a 30% increase in fats over their standard, low fat diet and Group 3 will have no increase in fats. Group 1 will receive high saturated fat supplements, Group 2 will receive balanced saturated/unsaturated fat supplements, and Group 3 will receive nonfat snacks. We will address the following specific aims. Specific Aim 1: To determine the effects of three diets differing in fatty acid quantity and balance on neuroendocrine, behavioral, and brain functional responses to reward processing in adolescent marmosets. We will utilize a separation social support test to determine neuroendocrine and behavioral responses to stress with and without social support and functional MRI to examine activation of specific brain regions related to memory, reward, and social recognition via familial olfactory cues. Specific Aim 2: To determine the effects of three diets differing in fatty acid quantity and balance on social behavior, activity levels, and problem solving and learning. We will use behavioral observations, activity measurements, and a visual discrimination reversal learning task to evaluate these key components of adolescent mood disorders. Alterations in components of the metabolic syndrome and puberty-related changes in steroid levels will be taken into account. These studies will provide important new information on the impact of dietary fatty acid composition on mood disorders in a primate.